The present invention relates to a design method and design apparatus for designing an antenna coil used in a non-contact IC card or a RFID.
In the non-contact IC card, the RFID, or the like (which, hereinafter, will be referred to as “IC card or the like”), an electromagnetic field (magnetic field) radiated by a reader/writer unit (hereinafter, referred to as “R/W”) is received by an antenna formed on the IC card or the like. Then, the electromagnetic field (magnetic field) received is converted into electric power so as to drive the IC. Also, communications from the R/W to the IC card or the like are performed using signal components contained in modulation or the like of the electromagnetic field (more essentially magnetic field in this case) radiated by the R/W. Moreover, in communications from the IC card or the like to the R/W, the signal transmissions to the R/W are performed by load modulation or the like.
At this time, the electric power which is receivable by the IC card or the like is determined based on the self inductances, mutual inductance, and coupling coefficient of the antenna formed on the IC card or the like and an antenna formed on the R/W. Furthermore, it is required that the electric power which is supplyable to the R/W antenna be suppressed within a range in which the electric field radiated from the R/W antenna satisfies the regulations by the Radio Law or the like.
This requirement determines an upper-limit of the electric power supplyable to the R/W antenna. In addition, the electric power to be received is determined from a transmission efficiency of the electric power to the IC card or the like. As a result, the communications area is determined.
Accordingly, it is required that electrical elements of the coil antenna to be formed on the IC card or the like, e.g., the antenna's inductance, resistance, and capacitance, be designed while satisfying various conditions by considering the coil antenna itself and its relationship with the R/W.
Meanwhile, as antenna design technologies and analysis technologies for the antenna design, there exist such methods as the moment method and finite-element method. These techniques, however, are the following methods: Configuration data on an antenna is inputted first, and then respective types of set conditions (i.e., boundary conditions and so on) are set for this configuration, and after that, the analysis is performed. A conventional antenna design method has been disclosed in, e.g., JP-A-7-22976. This conventional method is as follows: With respect to a frequency range necessary for the antenna design, frequency characteristics of respective elements of an impedance matrix in the moment method are stored into a memory in advance. This in-advance storage allows a significant reduction in the computation time necessary for the antenna design.